[LV] Remove scalable constraints on creating bitcasts

InnerLoopVectorizer::createBitOrPointerCast only supported fixed
length vectors since it hadn't been updated. Supporting scalable
vectors is just a matter of changing types and using elementcount
instead of numelements, since there's nothing which actually relies
on knowing the exact length of the vector.

Original written by mgabka.

Split out from D145163.